Liquid/liquid extraction of acidic aqueous streams comprising sulfate/titanium/iron values

ABSTRACT

Acidic aqueous phases comprising sulfate ion and particularly sulfuric acid values, titanium ion and particularly titanium (IV) values, and iron ion, particularly iron (II) values, and advantageously waste streams emanating from a sulfate process for the production of TiO 2 , are extracted with an initial organic phase which comprises at least one neutral organic extractant having the general formula: ##STR1## in which A and B, which may be the same or different, are the groups R 1  or OR 2 , wherein R 1  and R 2  are straight or branched chain alkyl, alkenyl, alkynyl, alkoxyalkyl, aryl or alkylaryl radicals, or halogen substituted such radicals; and R is either R 1  or R 2  as above-defined, with R, R 1  and R 2  either being the same or different, or R is a group having the general formula: ##STR2## in which Y is a straight or branched chain alkylene radical, and A and B are as defined above, to provide a final aqueous phase comprising substantially all of said iron (II) values present in said initial aqueous phase and a final organic phase comprising substantially all of said sulfuric acid values and substantially all of said titanium values present in said initial aqueous phase; the respective iron/sulfate/titanium values may then be facilely recovered from their respective final phases.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the extraction of acidic aqueous phasescontaining sulfate ions, in particular sulfuric acid, and additionallytitanium, iron and in particular iron (II) ions, and possibly othermetallic impurities. More especially, this invention relates to theextraction of solutions of residual sulfuric acid, which notably resultfrom the sulfate process for the production of titanium dioxide, torespectively recover the sulfate/titanium and iron values therefrom.

2. Description of the Prior Art

It is of course a truism that sulfuric acid is one of the most widelyused chemicals in industry. In many cases, the acid is dumped after use,which gives rise to pollution problems which become increasingly severeas the amounts of acid used increase.

Certain industrial processes and in particular the sulfate process forthe production of pigmentary titanium dioxide produce large amounts ofaqueous sulfuric acid, in a typical concentration by weight of from 10%to 50%, containing metal sulfates such as ferrous sulfates and titanium,as impurities. Such acid solution may also contain substantial amountsof other metal salts.

Said metal salts are in fact formed as by-products, for example, in theproduction of TiO₂, when the titaniferous raw material such as ilmeniteis subjected to attack by, or acidulation with sulfuric acid, such metalsalts being salts of aluminum, vanadium, chromium and manganese, inparticular.

The iron content, primarily in the ferrous state, is generally in therange of from 5 to 100 g/l, while the titanium content is from 1 to 20g/l.

It will be appreciated that the presence of one or more of theaforementioned metal salts in the residual aqueous acid gives rise tothe danger of causing problems in regard to dumping it into a river orother land waters, or into the sea.

It may therefore be necessary to recover such residual acids, and such arecovery operation is moreover advantageous since, particularly asregards the production of titanium dioxide, such acids may represent 33%of the acid employed for acidulation of the ore.

However, the technique of concentrating a residual acid such as topermit its reuse for acidulating ilmenite, for example, 65% H₂ SO₄, isseriously hindered by virtue of the precipitation of impurities, whichcauses the evaporator units to foul.

Moreover, it has also been found that it is an attractive prospect torecover the titanium contained in the residual acid. In fact, whenproducing titanium dioxide, the residual waters may contain a notinconsiderable portion of the production. For example, for a facilityproducing 180 t/day of TiO₂, between 5 and 25 t/day of TiO₂ contained inthe residual waters is discharged into the sea.

A number of processes for treating acid solutions containing metalimpurities by extraction by means of organic solvents such as neutralphosphoric acid esters and amines is known to the art.

However, it to date has proven impossible to develop a process enablingboth purification of the residual solution and the recovery of titaniumvalues therefrom.

SUMMARY OF THE INVENTION

Accordingly, a major object of the present invention is the provision ofan improved process adapted to perform that dual function outlinedimmediately hereinabove.

Briefly, the process of the present invention features treating anacidic aqueous phase comprising sulfate ions, in particular sulfuricacid, titanium ions, in particular titanium (IV), and iron ions, inparticular iron (II), by extracting said acidic aqueous phase with aninitial organic phase comprising at least one organic extractant havingthe following general formula: ##STR3## in which A and B are the groupsR₁ or OR₂, wherein A and B may be identical or different, and R₁ and R₂are straight or branched chain alkyl, alkenyl, alkynyl, alkoxyalkyl,aryl or alkylaryl radicals, or halogen substituted such radicals; and(i) either R is a radical of the same type as those defined hereinbeforein respect of R₁ and R₂, with R, R₁ and R₂ being identical or different;or (ii) R is a group having the general formula: ##STR4## in which Y isa straight or branched chain alkylene group, preferably containing from1 to 12 carbon atoms, with A and B being as above-defined; and thencerecovering a final aqueous phase comprising substantially all of the Fe(II) ions present in the initial aqueous phase and a final organic phasecomprising substantially all of the sulfuric acid and all of thetitanium ions present in said initial aqueous phase.

DETAILED DESCRIPTION OF THE INVENTION

More particularly according to this invention, it will be seen that theproposed use of a well-defined class of organic extractants permits thesimultaneous extraction and ultimate recovery of sulfuric acid andtitanium values and enables separation thereof from the metalimpurities, in particular iron (II).

The initial aqueous phase is therefore an acidic phase containingsulfate ions, in particular sulfuric acid, titanium ions, in particulartitanium (IV), and iron ions, in particular Fe (II). The aqueous phasemay be a residual acid phase resulting from a process entailing the useof sulfuric acid.

Generally, and in particular in the case of a residual acid phaseresulting from a process for the production of TiO₂ from ilmenite, thatphase may have a sulfuric acid concentration of from about 10% to 50% byweight, more particularly from 20% to 40%.

The titanium content typically ranges from about 1 to 20 g/l and inparticular from 3 to 15 g/l.

The proportion of iron sulfate, primarily ferrous sulfate, expressed interms of iron, is typically in the range of from about 5 to 100 g/l, andin particular from 10 to 70 g/l.

In addition, and as indicated hereinbefore, the initial aqueous phasemay contain metal ions of at least one of the following elements:aluminum, chromium, manganese, calcium and vanadium.

Also present may be such other elements as zinc, copper, magnesium,nickel, lead or arsenic, albeit same are present in smaller amounts,particularly in the case of residual acid solutions resulting from anilmenite attack process.

The initial acid aqueous solution may optionally be subjected, beforethe treatment/extraction according to the invention, to apre-concentration step with separation of the sulfates, in particularferrous sulfate, which crystallize out.

The initial organic solution comprises at least one organic extractantof the type indicated above, it being appreciated that the extractingagents may be admixed with each other.

The subject organic extractants are organophosphorus compounds of theneutral oxygen donor atom type. They may be of the phosphonate type,such as, for example, dibutyl butylphosphonate (DBBP), di-(2-ethylhexyl)2-ethylhexylphosphonate (DEHEHP), bis(β-chloroethyl)vinylphosphonate,tetraethyl decylene diphosphonate (C₂ H₅ O)₂ OP--CH₂ --(CH₂)₈ --CH₂--PO(OC₂ H₅)₂, tetraethyl butylene diphosphonate (C₂ H₅ O)₂ OP--CH₂--(CH₂)₂ --CH₂ --PO(OC₂ H₅)₂, and tetraisopropyl methyl methylenediphosphonate (iC₃ H₇ O)₂ OP--CH(CH₃)--PO(iC₃ H₇ O).

They may also be of the phosphinate type, such as, for example,dioctylmethylphosphinate.

The extractants may also be phosphine oxides, such as, for example,oxides of di-n-hexylmethoxyoctylphosphine (DHMOPO), tri-n-butylphosphine(TBPQ) and trioctylphosphine (TOPO).

In certain instances, it is advantageous to use an organic phase whichcomprises at least one organic extractant of the above-indicated type,in solution in a diluent.

In fact, because of their physical properties in particular, certainextractants cannot be used in the pure state for extraction of theinitial aqueous phase. In such a case, besides its solubilizing action,the diluent also has a favorable effect on the physical properties ofthe extractant or extractants, by reducing, for example, the viscosityor the density of the organic phase.

The diluents, which may be used alone or in the form of mixtures, may becompounds such as aromatic or aliphatic hydrocarbons, alcohols, organicacids, ethers, halogenated solvents, ketones and alkylphosphates.

The level of concentration of the extractant or extractants in thediluent may range from about 10% to 90% by weight. That level ofconcentration depends in particular on the physical properties which areto be achieved in respect of the initial organic phase.

The temperature at which the initial aqueous and organic phases arebrought into contact does not appear to be critical. The temperature maybe set by the process which gives rise to the residual aqueous phasethat is to be treated. The temperature may also depend on the nature ofthe extractant employed. In practice, the temperature will range fromabout ambient temperature to about 80° C.

The operation of bringing the above-mentioned two phases into contactand the extraction operation are effected in known manner in a knownapparatus of the mixer-settler or column type, for example.

The extraction itself is advantageously carried out continuously andcountercurrently, over a plurality of stages.

After the contacting operation, the product obtained is a final aqueousphase comprising substantially all of the Fe (II) ions present in theinitial aqueous phase and possibly substantially all of the ions ofother metal impurities, such as aluminum, chromium, manganese andcalcium, when such elements are indeed present in the initial acidicaqueous phase.

The organic phase contains substantially all of the sulfuric acid andall of the titanium ions present in the initial aqueous phase, whichhave been selectively extracted therefrom.

It will be appreciated that the process of the invention also makes itpossible to selectively extract a substantial proportion of elements ofhigh economic value, such as, for example, vanadium, from the residualinitial aqueous phase, when that phase indeed contains such elements.

The final organic phase can then be treated using any known means, forexample, by re-extraction or back extraction with water, to recover thedifferent elements that it contains, in particular sulfuric acid andtitanium.

Inorder to further illustrate the present invention and the advantagesthereof, the following specific examples are given, it being understoodthat same are intended only as illustrative and in nowise limitative.

EXAMPLE 1

A residual solution which, upon the pre-concentration thereof, had thecomposition as set forth below, was produced as a by-product of thesulfate process for producing titanium dioxide pigment by the attack oracidulation of ilmenite with H₂ SO₄ at a level of concentration of 65%by weight:

    ______________________________________                                        (i)      H.sub.2 SO.sub.4                                                                      523       g/l (38% by weight)                                (ii)     SO.sub.4.sup.2-                                                                       568       g/l                                                (iii)    Cl.sup.-                                                                              1.13      g/l                                                (iv)     Fe      25.8      g/l                                                (v)      Ti      9.12      g/l                                                (vi)     Al      4.02      g/l                                                (vii)    Mn      4.57      g/l                                                (viii)   Cr      272       mg/l                                               (ix)     V       751       mg/l                                               (x)      Zn      86        mg/l                                               (xi)     Cu      76        mg/l                                               (xii)    Ca      435       mg/l                                               (xiii)   Mg      81        mg/l                                               (xiv)    Ni      3.9       mg/l                                               (xv)     Pb      6.2       mg/l                                               (xvi)    As      0.4       mg/l                                               ______________________________________                                    

An extraction thereof was carried out utilizing differentorganophosphorus compounds of the family comprising phosphine oxides,phosphonates and phosphinates. Such extractants were employed in thepure state.

The operation was performed at a temperature of 25° C. (except whenusing TOPO and TBPO). One liter of residual acid solution was agitatedwith one liter of organic solvent. After settling and separation,analysis of the various aqueous and organic phases made it possible tocalculate the levels of removal of sulfuric acid and titanium from theaqueous phase.

These various results are summarized in the Table I which follows. Inregard to TOPO and TBPO, the temperatures specified were those at whicheach operation was effected.

                  TABLE I                                                         ______________________________________                                                 H.sub.2 SO.sub.4                                                                            Aqueous Phase                                                     Initial     concentration                                                     aqueous phase                                                                             after       Extent of                                  Solvents   concentration                                                                             extraction  removal                                    ______________________________________                                        DBBP       523 g/l     397 g/l         24%                                    DHMOPO     523 g/l     402 g/l         23%                                    Bis-(β-chloroeth-                                                                   523 g/l     365 g/l         30%                                    yl) vinylphos-                                                                phonate                                                                       TBPO 70° C.                                                                       523 g/l     293 g/l         56%                                    TOPO 50° C.                                                                       523 g/l     392 g/l         25%                                    DEHEHP     523 g/l     443 g/l         15%                                    TBP (tri-n-                                                                              523 g/l     408 g/l         22%                                    butylphosphate)                                                               Dioctylmethyl-                                                                           523 g/l     345 g/l         34%                                    phosphinate                                                                   ______________________________________                                                 Titanium                                                                        Initial     Final       Extent of                                  Solvents   concentration                                                                             concentration                                                                             removal                                    ______________________________________                                        DBBP       9.12 g/l    4.98 g/l        45%                                    DHMOPO     9.12 g/l    1.27 g/l        86%                                    Bis-(β-chloroeth-                                                                   9.12 g/l    7.38 g/l        19%                                    yl) vinylphos-                                                                phonate                                                                       TBPO       9.12 g/l    1.38 g/l        84%                                    TOPO       9.12 g/l    0.85 g/l        91%                                    DEHEHP     9.12 g/l    1.57 g/l        83%                                    TBP        9.12 g/l    9.05 g/l        <1%                                    Dioctylmethyl-                                                                           9.12 g/l    0.7 g/l         92%                                    phosphinate                                                                   ______________________________________                                                 Iron                                                                            Initail     Final       Extent of                                  Solvents   concentration                                                                             concentration                                                                             removal                                    ______________________________________                                        DBBP       25.6 g/l    24.05 g/l        6%                                    DEHEHP     25.6 g/l    24.75 g/l        3%                                    DHMOPO                                                                        Bis-(β-chloroeth-                                                        yl) vinylphos-                                                                phonate    25.6 g/l                    <5%                                    TBPO, TOPO                                                                    Dioctylmethyl                                                                 phosphinate                                                                   ______________________________________                                    

EXAMPLE 2

This Example employed an aqueous phase having the same composition asthat used in Example 1.

1 liter of the residual acid was contacted with 1 liter of variousneutral organophosphorus compounds. The latter were used either in theform of mixtures with each other, or in a state of dilution in anorganic diluent. The extraction operation was carried out at atemperature of 25° C.

Analysis of the various aqueous and organic phases enabled determinationof the levels of removal of H₂ SO₄ and Ti which were initially presentin the aqueous phase. The results are set forth in the following TableII. The proportions specified in respect of the mixtures are by weight.

                  TABLE II                                                        ______________________________________                                                              Extent of removal                                       Solvents                H.sub.2 SO.sub.4                                                                       Ti                                           ______________________________________                                        50% DBBP - 50% DHMOPO   27%      87%                                          80% DBBP - 20% DHMOPO   27%      85%                                          TOPO 35%, decanol 45%, kerosene 20%                                                                   18%      82%                                          TOPO 10%, DHMOPO 70%, kerosene 20%                                                                    21%      97%                                          TOPO 35%, versatic acid 25%, kerosene 40%                                                              8%      81%                                          TOPO 35%, Methyl isobutyl ketone 65%                                                                  14%      74%                                          TOPO 10%, DBBP 90%      27%      88%                                          ______________________________________                                    

EXAMPLE 3

This Example used two solutions of impure sulfuric acids resulting fromthe sulfate process for the production of titanium dioxide. Sample A wasof the typical composition of the mother liquors, while Sample B was anacid which had been subjected to a pre-concentration step withcrystallization of the ferrous sulfate which had been separated off.

    ______________________________________                                        Sample A          Sample B                                                    ______________________________________                                        H.sub.2 SO.sub.4                                                                     268     g/l (23%)  H.sub.2 SO.sub.4                                                                     523  g/l (38%)                               Fe     48.4    g/l        Fe     25.8 g/l                                     Ti     4.26    g/l        Ti     9.12 g/l                                     ______________________________________                                    

Each of the aforesaid samples was used to carry out a series of tensuccessive contacting steps in flasks, with fractions of dibutylbutylphosphonate. Operation was effected at a temperature of 25° C.

Operation was then continued in the same manner, but using TBP as thesolvent.

When using sample A, DBBP resulted in a raffinate containing 5% of H₂SO₄ by weight, while TBP resulted in a raffinate with 10.5% of H₂ SO₄.

When using sample B, DBBP resulted in a raffinate containing 5% of H₂SO₄ by weight while TBP resulted in a raffinate containing 13% of H₂SO₄.

It will therefore be seen that sulfuric acid is extracted with a muchhigher degree of efficiency when using DBBP than when using TBP.

EXAMPLE 4

The residual acid of Example 1 will now be considered.

A continuous extraction was carried out, using DBBP, over 6 theoreticalstages operating countercurrently, with a ratio between the flow ratesby volume of organic phase and aqueous phase, of approximately 3. Thetemperature was fixed at 25° C.

The organic phase was then subjected to a re-extraction operation usingwater over 6 theoretical stages, with a ratio between the flow rates byvolume of organic phase to aqueous phase of 3.

Table III reports the composition of the raffinate produced afterextraction by the organic phase, and the level of extraction.

It was found that there was a level of recovery of 91% in respect of H₂SO₄ and 92% in respect of titanium in the organic phase. On the otherhand, the other metal impurities: iron, aluminum, manganese, calcium andchromium remained for the most part in the raffinate.

Moreover, it will be appreciated that an economically attractiveelement, vanadium, was also extracted and recovered to a substantialdegree.

                  TABLE III                                                       ______________________________________                                                  Initial                                                             Composition                                                                             impure acid                                                                             Raffinate Extent of extraction                            ______________________________________                                        H.sub.2 SO.sub.4                                                                        523    g/l    78   g/l  91%                                         Fe        25.8   g/l    24.7 g/l  5%                                          Ti        9.12   g/l    0.9  g/l  92%                                         Al        4.02   g/l    5    g/l  <1%                                         Mn        4.57   g/l    5.6  g/l  8%                                          Cr        272    mg/l   330  mg/l 3%                                          V         751    mg/l   700  mg/l 27%                                         Ca        435    mg/l   520  mg/l 3%                                          ______________________________________                                    

While the invention has been described in terms of various preferredembodiments, the skilled artisan will appreciate that variousmodifications, substitutions, omissions, and changes may be made withoutdeparting from the spirit thereof. Accordingly, it is intended that thescope of the present invention be limited solely by the scope of thefollowing claims.

What is claimed is:
 1. A process for the treatment of an acidic aqueousphase containing sulfate ion and particularly sulfuric acid values,titanium ion and particularly titanium (IV) values, and iron ion,particularly iron (II) values, comprising (i) extracting said aqueousphase with an initial organic phase which comprises at least one organicextractant having the general formula: ##STR5## in which A and B, whichmay be the same or different, are the groups R₁ or OR₂, wherein R₁ andR₂ are straight or branched chain alkyl, alkenyl, alkynyl, alkoxyalkyl,aryl or alkylaryl radicals, or halogen substituted such radicals; and Ris either R₁ or R₂ as above-defined, with R, R₁ and R₂ either being thesame or different, or R is a group having the general formula: ##STR6##in which Y is a straight or branched chain alkylene radical, and A and Bare as defined above; and thence (ii) recovering a final aqueous phasecomprising substantially all of said iron (II) values present in saidinitial aqueous phase and a final organic phase comprising substantiallyall of said sulfuric acid values and substantially all of said titaniumvalues present in said initial aqueous phase.
 2. The process as definedby claim 1, wherein the initial organic phase comprises the at least oneorganic extractant in solution in a diluent.
 3. The process as definedby claim 2, wherein said diluent comprises at least one aromatic oraliphatic hydrocarbon, alcohol, organic acid, ether, halogenatedsolvent, ketone and alkyl-phosphate.
 4. The process as defined by claim2, wherein the degree of concentration of said organic extractant in thediluent ranges from about 10% to 90% by weight.
 5. The process asdefined by claim 1, said at least one organic extractant comprising aphosphine oxide.
 6. The process as defined by claim 5, phosphine oxidecomprising di-n-hexylmethoxyoctylphosphine oxide, tri-n-butylphosphineoxide or trioctylphosphine oxide.
 7. The process as defined by claim 1,said at least one organic extractant comprising a phosphonate.
 8. Theprocess as defined by claim 7, said phosphonate comprising dibutylbutylphosphonate, di-(2-ethylhexyl) 2-ethylhexylphosphonate orbis(β-chloroethyl)vinylphosphonate.
 9. The process as defined by claim1, said at least one organic extractant comprising a phosphinate. 10.The process as defined by claim 9, said phosphinate comprisingdioctylmethylphosphinate.
 11. The process as defined by claim 1, said atleast one organic extractant comprising admixture of dibutylbutylphosphonate with trioctylphosphine oxide ordi-n-hexylmethoxyoctylphosphine oxide.
 12. The process as defined byclaim 1, said at least one organic extractant comprising admixture oftrioctylphosphine oxide with methylisobutyl ketone, or with versaticacid and kerosene, or with decanol and kerosene, or with kerosene anddi-n-hexylmethoxyoctylphosphine oxide.
 13. The process as defined byclaim 1, wherein said initial aqueous phase further comprises vanadiumvalues, and said final organic phase comprises a substantial fraction ofthe vanadium values present in said initial aqueous phase.
 14. Theprocess as defined by claim 1, wherein said initial aqueous phasefurther comprises aluminum, chromium, manganese or calcium values, oradmixtures thereof, and said final aqueous phase comprises substantiallyall of such values.
 15. The process as defined by claim 1, wherein saidinitial aqueous phase is continuously and countercurrently extractedwith said initial organic phase.
 16. The process as defined by claim 1,wherein said initial aqueous phase is concentrated prior to extractionwith said initial organic phase.
 17. The process as defined by claim 1,wherein said final organic phase is re-extracted with water to recover athird aqueous phase comprising said sulfuric acid and titanium values.18. The process as defined by claim 1, said initial aqueous phasecomprising a solution of residual sulfuric acid emanating from a sulfateprocess for the production of titanium dioxide.
 19. The process asdefined by claim 1, wherein R is ##STR7## and Y contains from 1 to 12carbon atoms.
 20. The process as defined by claim 1, wherein saidinitial aqueous phase further comprises zinc, copper, magnesium, nickel,lead or arsenic values, or admixtures thereof.